The invention relates to a method and a device for measuring.
Distance measuring devices of this generic type are well known in the prior art. They have a distance measurement range of several tens of meters and are often embodied as handheld devices. They are used primarily in construction surveying or in indoor construction, for instance for measuring rooms three-dimensionally. Other areas of use for distance measuring devices are geodetic and industrial measurement and surveying. The fundamental principle of distance measurement with the known devices is based on the evaluation of a changeover time in a characteristic variable of the electromagnetic radiation emitted by the device and remitted by an object aimed at. The distance measuring device is equipped for that purpose with an emitter for emitting an intensity-modulated beam. In handheld devices, this primarily involves an optical beam in the visible wavelength spectrum, to make it easier to aim at the measurement points. The optical beam is remitted or scattered by the measurement object aimed at and is recorded by a receiver built into the device. From the chronological delay in the received modulated beam compared to the beam emitted by the emitter, the distance from the measurement object is obtained.
As detectors in the known distance measuring devices, PIN photodiodes or avalanche photodiodes are typically used for converting the beam, remitted or scattered by the measurement object, into electrical signals. Distance measuring devices in which the distance determination is based on the measurement principle of phase measurement are quite common. In such devices, the electrical reception signal has a mixer frequency superimposed on it, directly at the avalanche photodiode or downstream of a preamplifier, to produce a low-frequency measurement signal. The phase of this low-frequency signal is determined and compared with the phase of a reference signal. The difference between the measured phase of the low-frequency measurement signal and the phase of the reference signal is a measure of the distance of the measurement object.
In European Patent Disclosure EP-B-0 738 899, the behavior of laser diodes for visible radiation and the attendant problems of accuracy in distance measurement are described. To improve the accuracy of the distance measurement, it is proposed there that the emitted laser radiation be modulated with pulse widths of less than 2 ns. The modulation frequency of this known device is in the range of approximately 50 MHz. In pulses with a pulse width of 1 ns, for instance, and a period duration of 20 ns, a pulse power of approximately 20 mW is required in these known devices, if a mean power of 1 mW, which is still permitted in class 2 lasers used in these devices is to be attained. The proposed type of modulation can still be converted with the commercially available 3 mW lasers, without having to make major sacrifices in terms of the service life of the lasers from the increased pulse power compared to the continuous 3 mW mode. By means of the brief pulses and the high pulse power, a short coherence length of the laser radiation is attained. The result is a reduction in the generally granular intensity distribution of the radiation remitted by the usually rough surface of the measurement object aimed at. Granular intensity distribution is also known as speckles and affects the measurement accuracy that can be attained. By reducing speckles in the remitted measuring beam striking the detector, a measurement accuracy in the millimeter range is attainable.
One possibility of improving the existing methods and devices is to increase the modulation frequency of the measuring beam still further. At higher frequencies, the signal steepness at the zero crossover is greater, and thus the time interval Δt during which the intrinsic noise impairs the signal level is shortened. Shortening the time interval, however, has the disadvantage of reducing the measurement range within which the distance can be determined with the requisite accuracy. While at a modulation frequency of 50 MHz a distance of 3 m can be measured with the requisite accuracy, at a frequency of 400 MHz, for instance, the distance that can be measured with the requisite accuracy is only 37.5 cm. In distance measuring devices based on phase measurement, lower modulation frequencies of the measuring beam are additionally used for determining the approximate distance. At higher modulation frequencies for precision measurement therefore, increased effort and expense is required for measuring the approximate distance.